Process for the purification of molybdenum trioxide

ABSTRACT

THIS INVENTION PROVIDES A PROCESS FOR EXTRACTING IMPURITIES FROM A MOLYBDENUM OXIDE CONCENTRATE WHICH COMPRISES LEACHING THE CONCENTRATE WITH AN AQUEOUS SOLUTION OF AN AMMONIUM HALIDE AND A HYDROHALIC ACID.

United States Patent "ice 3,694,147 PROCESS FOR THE PURIFICATION OF MOLYBDEN UM TRIOXIDE James Louis Drobnick, Lakewood, and Tom Tse-Pu Chen, Arvada, Colo., assignors to Molybdenum Corporation of America, New York, NY.

No Drawing. Continuation of abandoned application Ser. No. 780,905, Dec. 3, 1968. This application Nov. 18, 1971, Ser. No. 200,234

Int. Cl. (32% 59/00 US. Cl. 423-53 3 Claims ABSTRACT OF THE DISCLOSURE This invention provides a process for extracting impurities from a molybdenum oxide concentrate which comprises leaching the concentrate with an aqueous solution of an ammonium halide and a hydrohalic acid.

This is a continuation of application Ser. No. 780,905, filed Dec. 3, 1968 and now abandoned.

This invention relates to a novel process for the extraction or removal of impurities from molybdenum oxide concentrates and more particularly to a process involving leaching such concentrates with an aqueous mixture of: (a) an ammonium halide; and (b) a hydrohalic acid such as hydrochloric.

Molybdenum oxide is produced commercially from impure molybdenum sulfide by subjecting the sulfide to high temperature calcination, e.g. at temperatures of from about 540 C. to 640 C. The calcination of the molybdenum sulfide is conducted in hearth roasting or rotary kiln pyrometallurgical equipment to drive off substantially all the sulfur as sulfur dioxide. This leaves a major portion of the concentrate as molybdenum trioxide, M00 also simply referred to herein as molybdenum oxide. The impure molybdenum sulfide is, in turn, recovered from ores using benefication techniques, such as flotation. However, during the flotation process for the recovery of molybdenum sulfide, it is practically impossible to quantitatively eliminate other naturally occurring sulfides. Although the calcination removes substantially all the sulfur, other impurities from the naturally occurring sulfides such as those of lead, copper, iron and zinc, and in some cases alkaline earths such as calcium and magnesium, remain in the molybdenum oxide concentrate. These impurities or contaminants are deleterious for various uses of the molybdenum oxide and should eventually be removed, e.g. prior to use in the steel industry. Thus, the lead impurity, if incorporated into steel along with the molybdenum, interferes with steel processing, particularly in the rolling mills because of brittleness.

It is known that commercially available mineral acids can be used as leaching reagents in aqueous systems for the extraction of impurities in molybdenum oxide concentrates. However, the acids are limited to hydrochloric acid and nitric acid since sulfuric acid would not remove the lead impurity due to the formation of insoluble lead sulfate. Furthermore, the yield of molybdenum values resulting from hydrochloric acid or nitric acid leaching are poor due to the high solubility of molybdenum values in the acidic solution.

It has now been found that impurities can be removed from a molybdenum oxide concentrate without loss of ex-,

cessive quantites of molybdenum values by leaching the concentrate with an aqueous solution of a hydrohalic acid and ammonium halide. The ammonium halide suppresses the solubility of molybdenum in the acidic leaching solution and at the same time enhances the solubility of contaminants such as lead.

3,694,147. Patented Sept. 26, 1972 The leaching process of this invention is highly effective in removing impurities from molybdenum oxide concentrates while at the same time depressing the molybdenum solubility. This process shows but small losses of molybdenum and thus permits high yields of molybdenum trioxide. Illustratively, the molybdenum loss of concentrates subjected to the leaching can be kept to below 1% or 2% or even less than 0.6%

The use of the leaching solution of this invention overcomes the problem of low molybdenum yields by hydrochloric acid or nitric acid leaching. This, in turn, eliminates secondary recovery for the substantial quantities of solubilized molybdenum following the impurity removal step. Furthermore, the leaching solution of this invention removes lead impurities which are not afiected by sulfuric acid leaching.

Illustrative of impurities which can be removed from molybdenum oxide concentrates by the process of this invention there can be mentioned those of lead, copper, iron, zinc and alkaline earths such as calcium and magnesum.

The process of this invention is particularly suitable for the removal of lead impurities. Preferably the leaching operation is conducted so that the purified molybdenum oxide contains less than 0.01% lead.

The coaction of the hydrohalic acid and ammonium halide appears to be fairly specific. The halide salts that may be used according to the present invention include ammonium chloride and ammonium bromide, with the ammonium chloride being preferred. Other halide salts in place of an ammonium halide in the leaching solution of this invention, have been investigated and found to be unsuitable. Thus, leaching of a molybdenum oxide concentrate with a combination of hydrochloric acid and sodium chloride or calcium chloride in aqueous solution failed to achieve the low molybdenum solubility found with the leaching solution of this invention. Furthermore, leaching of molybdenum oxide concentrates with a combination of nitric acid and ammonium nitrate in aqueous solution failed to give satisfactory lead impurity removal.

The process of this invention is conducted by intimately contacting the impure molybdenum oxide concentrate with the leaching solution. Evenutally, the liquid containing the solubilized or extracted impurities is separated from the leached solids. Generally, the leaching solution and molybdenum oxide concentrate in finely divided form, e.g. a particle size passing through about a No. 35 sieve, of the standard screen scale, will be admixed to provide the intimate contact.

The leaching solution of this invention will contain a sufiicient quantity of ammonium halide to depress the molybdenum solubility. Ordinarily, the leaching solution will contain, dissolved therein, at least 25 grams of ammonium chloride, per liter of the solution.

However, the leaching solution can contain sufiicient ammonium chloride to substantially saturate the solution at the leaching temperature. Desirably, the leaching solution will contain from about 50 to about 500 grams (g.) per liter (1.) of the ammonium chloride.

The quantity of hydrohalic acid in the aqueous leaching solution can vary over a wide range, sulficient to dissolve impurities in the concentrate. Ordinarily, there can be provided or charged to the leaching solution from about 0.05 pound to about 0.25 pound of hydrohalic acid, preferably hydrochloric acid, per pound of the molybdenum in the molybdenum oxide concentrate. However, within this range, the quantity of acid will desirably vary dependent on the amount of impurities present so that the quantity of acid is in slight excess, e.g. 10% excess, over the stoichiometric amount required to react with the impurities.

It will, of course, be recognized that during the leaching operation, hydrohalic acid can be added to the aqueous leaching solution e.g. when leaching solution is recirculated, to compensate for depletion of acid by reaction with the concentrate and with basic chemicals which may find their way into the solution, e.g. ammonia or ammonium salts used to purify recycled leach filtrate as shown in Example hereinafter. Alternatively, the leaching solution can be made up, e.g. in batch processing, so that it contains the entire desired quantity of hydrohalic acid and ammonium chloride. Ordinarily, however, a concentration of from about grams to that of over 150 grams and preferably from about grams to 100 grams of the hydrohalic acid per liter of the aqueous leaching solution can be employed. References to hydrohalic acid herein, e.g. hydrochloric acid, are to the hydrogen halide, e.g. HCl. It can be provided in the leaching solution by conventional means, e.g. by addition or mixing of commercial highly concentrated aqueous solutions of the hydrohalic acid. Illustrative of hydrohalic acids which can be used in the process of this invention, there can be mentioned hydrochloric acid and hydrobromic acid. Furthermore, the various hydrohalic acids can be combined with ammonium chloride in preparing the leaching solution.

The pH of the leaching solution should be sufiiciently low to permit solubilization of lead and other impurities such as those mentioned hereinabove. Ordinarily the pH of the leaching solution will be below 1.5 e.g., 0.5, al-

though higher pH values of the leaching solution, e.g. a pH of below 2 could also be employed. The pH of the leaching solution can be easily lowered by increasing the concentration of hydrohalic acid. If desired, the pH can be raised to the above noted values by the addition of ammonia.

The leaching step can be carried out with conventional techniques and conventional equipment employed for acid leaching. The ratio of solids to leaching solution can vary over a wide range depending on the particular equipment and techniques employed, the quantity of impurities in the concentrate, the desired level of purity of the final product, as well as the concentration of the ammonium chloride and hydrohalic acid in the leaching solution. A sufficient quantity of solution should be provided to thoroughly wet the concentrate. However, it is convenient to use a ratio of about 1 part of solids for each part of the solution to about 1 part of solids for each of three parts of the solution and particularly about 2 parts of solids for 3 parts of the solution.

Ordinarily, the leaching operation of this invention will be performed at elevated temperatures such as that of from about 50 C. to about 90 C. but preferably at temperatures of from about 60 C. to about 80 C. The time of contact of the leaching solution with the concentrate can vary over a wide range dependent, again, on considerations mentioned hereabove, e.g. temperature, quantity of impurities, concentration of reagents in the leaching solution, and the like. Thus the leaching operation for any portion of the concentrate can vary from a few minutes, e.g. 15 minutes to a few hours, e.g. three or more hours. Ordinarily, any particular portion of the concentrate will be subjected to leaching for about 30 minutes to one hour.

Following the leaching operation the liquid containing leached impurities and the leached solids can be separated by conventional liquids-solids separation procedures, such as filtration. The solids, or residue of purified M00 product can then be dried and calcined to remove ammonium chloride and other volatile materials picked up during the processing.

Conventional molybdenum oxide concentrates can be employed as the feed. Molybdenum oxide concentrates suitable for use in this invention have been described hereabove in the discussion of the commercial preparation of molybdenum oxide. Technical molybdic oxide can be used in this invention. Such a concentrate contains fairly sizable quantities of impurities as can be seen from the analysis of what is known as technical molybdic oxide and which contains only about 79% to 90% of M00 with an M0 equivalent of 52.5-60% and includes impurities mentioned hereinbefore, e.g. as much as 0.5% of copper. However, molybdenum oxide concentrates having substantially greater purity than technical molybdic oxide, e.g. those containing or more of M00 can be used. Also, concentrates containing less of the molybdenum oxide, e.g. 75% of M00 can be employed.

The reference to parts or percentages herein is on a weight basis.

The following examples are illustrative of the invention.

EXAMPLE 1 Four experiments were made with molybdenum oxide concentrate containing 59.2% molybdenum, 0.14% lead, 0.34% iron and 0.11% copper. In each of the experiments the concentrate was leached with an aqueous solution containing: (a) 200 grams per liter of ammonium chloride; and (b) varying amounts of hydrochloric acid, as shown below in Table 1B. The leaching was conducted for a period of one hour at a temperature of 55 C. to 60 C. The pH of the solution was below 1.5 during the leaching operation. The ratio of concentrate to aqueous leaching solution was 1 to l. The impure molybdenum oxide concentrate had an average particle size of less than 48 mesh. Table 1A, below, gives the weight, in grams, of the molybdenum oxide concentrate, leaching solution, hydrochloric acid and ammonium chloride employed in each of the four experiments. Following the leaching step, the slurry containing the purified concentrate and the liquids containing contaminants were separated by filtration. Both the filtrate and purified concentrate were analyzed for lead and iron and the molybdenum loss of the concentrate was determined. The results of the experiments are presented in Table 1B wherein column A gives the experiment number, column B gives the grams of hydrochloric acid in the solution per gram of molybdenum in the concentrate. The column headed Pb gives the percentage of lead extracted, and the column headed Fe gives the percent of iron extracted.

TABLE 1A Grams Ooncen- Leaching HCL Experiment trate solution NH4C1 TABLE 1B Leach residue analysis, percent Mo loss, A B Pb Fe percent Pb Fe It can be seen from the above Table 1B that when a sufiicient quantity of hydrochloric acid is used, both the lead and iron impurities were markedly decreased with but a low molybdenum loss.

EXAMPLE 2 Experiments were run in substantially the same manner as that of Example 1 with the exception that the aqueous leaching solution, also simply referred to herein as the lixivant, was changed to an aqueous solution of nitric acid, in place of hydrochloric acid, and ammonium nitrate was substituted in place of ammonium chloride. Two different molybdenum oxide concentrates were used. One of the concentrates was the same as that used in Example 1 and is referred to as A. It contained 0.14% Pb and 59.2% M0. The other concentrate contained 0.24% lead and 59.0% molybdenum and is referred to as B. Table 2 TABLE 2 Nitric acid usage, IIL/LIIF.

Leach residue analysis, Pb extraction M003 concentration percent P b percent A (0.14% Pb) 0.15 0.11 25. 9

B (0.24% Pb) 0. 0. 235 3. 6

EXAMPLE 3 Comparative experiments were made in order to determine impurity removal and molybdenum loss of molybdenum oxide concentrate by the use of the HCl-NH CI leaching solution of this invention in comparison with the ,use of an aqueous solution of hydrochloric acid with difi'erent chloride salts. The molybdenum oxide concentrate employed in this example contained 59.0% Mo, 0.26% Pb, 0.32% Cu and 3.1% Fe. Each of the leaching solutions (or lixivants) contained 150 grams of the designated chloride salt per liter. The HCl used was 0.15 lb./lb. of M0 in concentrate. Thus the solids to liquid ratio was about 3.5:6.5. The leaching was performed for one hour at a temperature of 40-60 C. The pH of the leaching solutions was below pH 1.0. The leaching was conducted in glass beakers using a heated magnetic stirrer for agitation. Each of the three slurries of purified molybdic oxide and lixivant containing matter dissolved out of the concentrate were separated by filtration. The purified molybdenum oxide residues were analyzed for lead, iron and copper impurities. The results of the analysis are shown in Table 3. The filtrates were also analyzed for molybdenum content and this is expressed as Molybdenum Solubility in Leach Solution, on a gram per liter basis in Table 3.

It can be seen from Table 3 that the combination of hydrochloric acid with ammonium chloride suppressed the solubility of molybdenum while extracting the lead, iron and copper contaminants. On the other hand, large quantities of molybdenum were found in the leach solution when the halide salt was that of sodium or calcium.

Two different molybdenum oxide concentrate samples were used to investigate the amount of ammonium chloride required in the aqueous leaching solution for adequate lead removal as well as the eifect of variation in lead content of different molybdenum oxide concentrates. The molybdenum solubility in the aqueous leaching solution (lixivant) as a function of ammonium chloride was also investigated. The two samples of concentrate used in the investigations are designated as Sample A and Sample B. Sample A contained 59.0% molybdenum and 0.26% lead. Results with this sample are shown in Table 4A below. Sample B contained 59.2% molybdenum and 0.14% lead. Results with this sample are shown in Table 4B below. In each experiment 150 milliliters of the lixivant were used. Each lixivant was composed of an aqueous solution of: (a) hydrochloric acid (in the amount shown in Tables 4A and 4B); and (b) for experiments 37, ammonium chloride in the concentrations shown in the tables. The lixivants of Experiments 1 and 2 were simply aqueous solutions of hydrochloric acid without any ammonium chloride. grams of the concentrate amounting to 40% solids was used in each experiment. The pH in each experiment was less than 2.0. The slurry was filtered after the leaching step. The amount of lead remaining in the leached residue was analyzed. Also the amount of molybdenum which dissolved into the filtrate was analyzed. The results of these experiments are shown in Tables 4A and 4B. It can be seen that an ammonium chloride concentration as low as 50 grams per liter is effective in assisting in the removal of lead impurities. Also, it can be seen, particularly from Table 4B, that as the ammonium chloride concentration was increased, the lead impurity remaining in the residue was decreased.

TABLE 4A 1 M00; Leach residue N H401, concen- H01 per M0 loss, analy s, Exp. g./l. trate lb. of Mo percent percent Pb 1 0 A 0.10 0. 02 0. 15 2 0 A 0. 20 0. 53 0. 05 3 200 A 0. 10 0. 09 0. 012 4 200 A 0. 10 0. 27 0. 014

TABLE 4B M00; Leach residue NHiCl, concen- R01 per Me less, analysis, Exp. g./l. trate lb. of Mo percent percent Pb 5 50 B 0. l0 0. 23 0. 024 6 100 B 0. 10 0. 20 0. 012 7 200 B 0.10 0. 19 0. 006

EXAMPLE 5 This example illustrates a continuous pilot plant process for the purification of molybdenum oxide concentrate by using an aqueous leaching solution containing HCl and NH Cl. The pilot plant consisted of (l) a variable-speed dry solids screw feeder, (2) a small 3-gallon mix tank, (3) three 15-gallon heated (direct live steam lances of PVDC) leach tanks, .(4) an 18-inch diameter x 12-inch face drum filter, (5) a 4-gallon continuous ammonia precipitation tank, (6) a 3-foot square pan filter and several miscellaneous surge tanks and pumps. The leach tanks were operated at 60 to 80 C., but no additional heat was applied to the product filter pan. The product on the drum filter was washed with ammonia precipitation filtrate and then with water.

The combined filtrates then passed through a surge tank system from which the 20% bleed was metered as well as the 20 to 60% fraction to the ammonia precipitation system. The ammonia precipitation consisted of a 4-gallon agitated tank to which anhydrous amomnia was continuously added to maintain a pH of 6.0 to 7.0. The resulting slurry was then passed to the 3-foot pan filter where batch filtration was performed. The filter cake was washed with a minimum of water (to maintain a water balance) and sampled. The filtrate was transfered to a surge tank where a small amount of HCl was added to prevent precipitation of ferric hydroxide, as air oxidation of ferrous chloride occurred.

The process was operated continuously for hours. The average time during which any portion of the concentrate was leached averaged about 40 minutes. The pH of the leaching solution was maintained at a pH of below 1.0. The ammonium chloride concentration of the leaching solution was maintained between that of 88 grams per liter to 236 grams per liter. The leaching was conducted at a 40% solids level on ground and screened feed of molybdenum oxide concentrate. Ninety seven percent of the feed had a particle size of less than 48 mesh. The hydrochloric acid provided in the leaching solution varied from 0.115 to 0.15 pounds per pound of M0 in the concentrate feed. Hydrochloric acid was added continuously. The temperature of the leaching solution was maintained between 6080 C. A 20% bleed stream of the filtrate was used to control impurity build-up. A portion of the filtrate was processed with anhydrous ammonia to also control build-up of impurities as well as provide recycle ammonium chloride to the leaching system.

The amount of filtrate subjected to ammonia precipitation ranged from 20 to 60 percent of the total filtrate. The molybdenum loss averaged 0.57% for the entire 155 hour concentration. The fed analysis of the molybdenum oxide concentrate as well as analyses of the leached residue is given in Table 5. Column A in Table sets forth the cumulative hours of operation of the leaching process. The column headed HCl Charge is on the basis of pounds of hydrochloric acid per pound of molybdenum in the concentrate.

Impurities, in addition to those set forth in Table 5, analyzed in the impure concentrate and in the leached residue are given in Table 6.

TABLE 6 Analysis, percent Leach Element Feed residue Ca 0.12 0.01 Mg 0. 17 0. 02 Zn 0.17 0.018

The purified residue of molybdenum oxide concentrate from the leaching step of Example 5 was calcined to remove chlorine and nitrogen impurities imparted by the processing of Example 5. Table 7 illustrates the efiectiveness of the heat treatment for the chlorine and nitrogen elimination.

TABLE 7 M00; analysis, percent Calclnation 1 hour retention 2 hour retention temperature, 0. 01 N Cl N What is claimed is:

1. A process for purifying molybdenum oxide concentrate containing more than 0.05% of a lead impurity, which process comprises:

(A) Leaching said concentrate with an aqueous solution of hydrochloric acid in an amount such that the pH is less than 2.0 and sufficient to dissolve said lead impurity in combination with ammonium chloride in a concentration greater than 25 grams per liter and sufficient to depress solubility of molybdenum in the leaching solution, whereby the solubility of molybdenum in the acidic leaching solution is suppressed resulting in a loss of molybdenum of less than 2%, and the solubility of lead impurity is enhanced so that the purified molybdenum oxide of step (B) contains less than 0.01% lead;

(B) Separating the liquids of the leaching solution from the purified leached molybdenum oxide solids; and

(C) Drying and calcining said molybdenum oxide solids to obtain a purified molybdenum oxide.

2. The process of claim 1 wherein the molybdenum oxide concentrate prior to the leaching step (A) is in finely divided form; the hydrochloric acid concentration during the leaching step (A) is such that the pH is less than 1.5 and the concentration of ammonium chloride is from 50 to 500 grams per liter; the loss of molybdenum during the leaching step (A) is less than 0.6%; the ratio of solids to solution in the leaching step (A) is from one part solids for each part of the solution to one part of solids for each of three parts solution; the leaching step (A) is carried out at a temperature from 50 C. to about C.

3. The process of claim 2 which consists essentially of steps (A), (B) and (C) so that secondary recovery of solubilized molybdenum is eliminated and wherein the time of the leaching step (A) is such that any particular portion of the molybdenum oxide concentrate is subjected to leaching for thirty minutes to one hour; in step (B) the liquids of the leaching solution are separated from the purified leached molybdenum oxide solids by filtration; and in step (C) the dried molybdenum oxide product is calcined between 250 C. and 600 C.

References Cited UNITED STATES PATENTS 1,838,767 12/1931 Iredell 23-15 W 1,923,652 8/1933 winklcr et al. 23-15 W 2,096,847 10/1 937 Donahue et a1. 23-15 W 2,965,447 12/1960 Zimmerley et al. 23-140 3,393,971 7/1968 Vanderpool et al. 23-22 OTHER REFERENCES Chemical Abstracts, 1959, p. 17653c.

HERBERT T. CARTER, Primary Examiner US. Cl. X.R. 23-22, 23, 98 

